Laboratory Evaluation of a Low pH and Low Polymer Concentration Zirconium-CMHPG Gel System for Hydraulic Fracturing
Low pH fracturing fluids, such as the transition metal cross-linked guar derivative, can reduce the formation damage caused by clay swelling and fines migration in hydraulic fracturing treatments. However, a major disadvantage of the fluids is the uncontrollable rapid viscosity development, especially cross-linking under a pH below 3.0. In this paper, a novel delayed organic zirconium cross-linker with lactate and ethylene glycol as ligands was designed to overcome the shortcoming. Carboxymethyl hydroxypropyl guar (CMHPG), which results in a low gel residue after breaking, was used as the polymer at low concentration. The impact of gelation parameters on the properties of zirconium-CMHPG gel system was systematically investigated using a Sydansk bottle-testing method combined with a controlled stress rheometer. Desirable gelation time delay with agreeable rheological performance could be obtained by optimizing the polymer and cross-linker concentrations even at a pH as low as 2.0. Uniformly and compactly generated 3D network microstructures could firmly lock the free water within the gel, which further improved the thermostability and acid-resistance of the system. Meanwhile, the structures could also maintain the viscoelasticity of the fluid at a high level to ensure the efficient transportation of the proppants. Proppant settling and gel breaking tests demonstrated that the low pH and low polymer concentration gel system generated fewer insoluble residues with an uncompromised suspension performance. Moreover, the formation damage evaluation revealed that the low pH fracturing fluid system produced a better return permeability than the high pH system.
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